Increased efficiency method for lining a pipe with a cement mortar

ABSTRACT

An increased efficiency method and apparatus for lining a section of pipe with a cement mortar. After a wet cement mortar has been applied to a cleaned interior surface of a pipe, carbon dioxide is introduced into the freshly lined section of the pipe in such quantity that a crust of calcium carbonate will be formed on the interior surface of the wet cement mortar lining as the carbon dioxide reacts with calcium compounds of the wet cement mortar in a relatively short period of time. This crust is provided to protect the underlying mortar from scouring, while tolerating a low-velocity flow of water, until the normal hardening of the underlying mortar through hydration can take place so as to substantially reduce the out-of-service time of the section of pipe which is being lined. In order to provide a saturated carbon dioxide gas condition within the water main section for effective and rapid formation of the crust but with a minimum waste of carbon dioxide gas, the carbon dioxide gas pressure is regulated to maintain a positive pressure just slightly greater than atmospheric, bellows preferably provided to precisely indicate pressure.

This application is a divisional of application No. Ser. No. 08/122,997,filed Sep. 17, 1993 (now U.S. Pat. No. 5,443,377), which is acontinuation in part of application Ser. No. 07/792,138, filed Nov. 13,1991 (now U.S. Pat. No. 5,246,641), the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an increased efficiency method andapparatus for lining a section of pipe with a cement mortar.

BACKGROUND OF THE INVENTION

A significant proportion of the water systems supplying potable water inthe major U.S. municipalities were built between 1850 and 1950. Thesubsurface water mains which convey the water from its source to theconsumer were made almost exclusively of ferrous metals such as steeland cast iron. With rare exceptions these pipelines were placed inservice with little or no protection against internal corrosion. Therare exceptions included the early use of Portland or natural cementmortar to line the inside of certain water mains before they were placedunderground. Subsequent experience has demonstrated that such cementmortar linings were by far the most effective means of permanentlyprotecting water mains against internal corrosion.

Among the serious problems caused by internal corrosion are a reductionin carrying capacity, perhaps as high as 50 to 75 percent of thepotential capacity, and rusty (red) water being carried to consumers'taps. The reduction in capacity is the result of the growth ofbarnacle-like nodules known as tubercules on the inside wall of thepipe. The tubercules cause turbulence which, in combination with thereduced cross-sectional area of the pipe, reduces the hydraulic carryingcapacity of the water mains. Cleaning the mains will return the capacityto its original value temporarily, but it will return to the reducedvalue in a very short time. The rusty or red water is the result ofcorrosion products being carried in the water stream.

While some individual pipe lengths have been factory cement mortar linedsince the mid-1800's, the general use of cement mortar lining was notfirmly established until about 1945. Today nearly all cast or ductileiron pipe is cement mortar lined at the factory as a part of themanufacturing procedure.

During that period when water mains were being installed before therecognition of the importance of cement mortar lining, many miles ofpipe were installed. Repeat cleaning was about the only means ofmaintaining water system capacity.

In the mid-1930's Albert G. Perkins developed a process for cementmortar lining pipelines in place. Several patents in the 1930's andearly 1940's were granted as a result of the development of the liningprocess, examples being U.S. Pat. Nos. 1,988,329 and 2,168,917. As shownin both of these patents, a wet cement mortar is thrown centrifugallyfrom a rotating distributor head out against the inner surface of thepipe. When properly applied, the mortar will adhere to the pipe surfaceuntil it hardens whereupon it forms a unitary self-supporting structurewithin the pipe. The Perkins patents also show rotating trowels mountedbehind the distributor head to smooth the lining material before ithardens. Additionally, U.S. Pat. No. 1,988,329 shows a rotating sprayhead through which asphaltum curing control liquid is pressure sprayedonto the freshly trowelled lining. U.S. Pat. No. 2,168,917 shows analternative means for applying a curing control liquid, namely a secondelectrically driven centrifugal distributor head. The application of theprocess shown in the foregoing patents was limited to pipelines whichwere sufficiently large in diameter to permit manual operation of themachines within a pipeline. The minimum diameter which could be linedwas about 24 inches.

In the early 1950's a process was developed which permitted the liningof mains of less than 24 inches in diameter, too small for a man toenter. U.S. Pat. No. 2,704,873 was granted to K. K. Kirwan and Alfred G.Perkins, one of the applicants of the present application, on thisprocess.

Since that time the process has been refined and additional patents havebeen granted, for example, U.S. Pat. Nos. 2,758,352; 3,044,136;4,067,680; and 4,252,763. Today pipelines as small as four inches indiameter are lined in place.

A conventional water main is indicated generally at 10 in the drawings.As is well known in the art, if the water main is of a ferrous materialand has not been coated prior to being placed into service, it willeventually become corroded and tuberculated. Thus, tubercules willgradually form on the interior wall of unprotected metallic water linesto reduce the hydraulic carrying capacity of a water transmission anddistribution system. Such water lines can be restored to "new pipe"hydraulic carrying capacity by cleaning and cement lining. In practicingthe prior art, temporary distribution lines 12 are installed abovegroundto maintain water service to residential and commercial customers.Access to the water main or pipe 10 requires excavation and removal ofsections of the pipe, typically 5 feet long. The distance between accesspoints 14 varies greatly in different water main layouts. Typically,they would be several hundred feet apart, the normal distances between400 to 800 feet. Sections of pipe which are not being cleaned and linedare closed off by existing line valves (if available) or alternativelyby temporary plugs 16. The section of pipe which is to be lined then hasthe tuberculation 18 removed by passing a pipe cleaner assembly (notshown) through the pipeline until all of the tubercular deposits havebeen removed. The cleaner is either mechanically winched through thepipe section or is propelled by water flow. Water passes through thecleaner to flush solid debris ahead of the unit. The cleaning processmay also include pulling a tight fitting circular squeegee or rubberswab (similar to squeegee 19 in FIG. 2) through the section of pipe toremove standing water and remaining loose solids. They may be pulledthrough the pipe separately, and they may be pulled through the pipeimmediately before the lining machine. In any event, the essentiallybare pipe is then ready for cement mortar application.

In the prior art illustrated in FIG. 1 a relatively small diameterpipeline is shown, that is, one having a diameter of less than 24inches. In pipes having a larger diameter, access is typically throughexcavated points, and manually operated equipment is used within thewater main, such equipment being illustrated in U.S. Pat. Nos. 1,988,329and 2,168,917. For cleaning relatively small diameter pipe, a remotelycontrolled pipe-lining apparatus such as the type shown in U.S. Pat. No.2,704,873 is utilized. In this form of apparatus dry mortar formed ofPortland cement 20 and sand 21 is mixed with water in a mixer 22 tostrict specifications and is then pumped by a pump 23 to a liningmachine 24 through a hose 26 mounted upon a hose reel 28. The liningmachine, which is also referred to as a centrifugal applicator, ispositioned at the end of the section of the pipe remote from the winch30 prior to being winched through the pipeline. The centrifugalapplicator may be followed by a flexible cone-shaped drag trowel 32,such as that shown in U.S. Pat. No. 4,184,830. The apparatus is thenwinched through the line 10 by operation of the winch 30 and winch line34, the apparatus throwing a wet cement mortar 36 onto the cleanedsurface of the water main 10. The foregoing method and apparatus is wellknown in the art.

In the application of the lining processes referred to, it has beennecessary to interrupt individual consumer services as a result of theremoval of a pipeline from services. Standard practice has been toprovide aboveground temporary piping 12. This has resulted in direct andindirect expenses and delays which are reflected in substantially higherprices than might otherwise be justified.

A properly applied lining may last perhaps 100 years, while an unlinedpipeline must be cleaned perhaps every six months. In order to avoid thecost of cement mortar lining in place and to reduce the out-of-servicetime, many utilities are nevertheless continuing the practice of repeatcleaning because initial cost is less and because its speed ofcompletion allows for a tolerable interruption of customer service sothat temporary aboveground service piping is not required. While thisprocedure is initially cost and time effective, when one considers thatit must be repeated again and again, it loses its cost and timeeffectiveness. In order that the interruption of service during liningmay be tolerable, it is desirable to be able to re-introduce water intoa water main within about two hours of cement mortar lining so that therehabilitation of a length of main may be completed within a workingday. This can be done if the cement mortar can be provided with a crustwhich is hardened sufficiently in 1 hour after lining to receive waterwithout erosion damage.

Various proposals have been made for reducing the out-of-service timewhen a cement lining is applied. Seal coats, such as disclosed in U.S.Pat. No. 4,252,763, have been used in factory cement mortar-lined castand ductile iron pipes for retaining residual moisture for curing andfor helping prevent excessive cracking due to moisture loss. They alsohelp to conceal imperfections and cracks in the cement mortar lining.The seal coats present barriers between the mortar and water carried inthe pipe to retard the rate at which soluble elements (primarily calciumhydroxide), which may adversely affect water quality by imparting a highalkalinity (high pH) to the water for the first few weeks after a newcement mortar lining is placed in service, are dissolved from thesurface of a new lining. However, such seal coats do not affect thechemistry of hydration of the cement in the mortar and therefore do notaccelerate its set time.

As pointed out in U.S. Pat. No. 4,252,763, the application of anasphaltum curing control liquid, as in earlier patents such as U.S. Pat.Nos. 1,988,329 and 2,168,917, was not satisfactory because the freshlyapplied wet mortar does not strongly adhere to the pipe surface, and, ifthe lining is disturbed to any substantial extent before it has takenits initial set and has begun to acquire structural strength of its own,it is likely to result in progressive erosion wherein the entire mortarlining pulls away and falls to the bottom of the pipe. In order toovercome these problems, U.S. Pat. No. 4,252,763 proposed to atomize theliquid so that a fine mist of the "curing compound" was applied to thesurface of the wet mortar. It is difficult to effect adherence of thisseal coat to a fresh lining since it tends to flake off into the water.Moreover, it does not accelerate its set time or provide resistance toscouring by early introduction of flowing water.

Another proposal which has been suggested is to use accelerators withinthe mortar mix. The use of accelerators has its limitations because ofthe complexity of the lining procedure which, after the mixing of allthe ingredients of the mortar in a mixer, requires the mixture to bekept a certain amount of time in a pump hopper. The cement mortar isthen pumped through a hose, which is typically 1 to 2 inches indiameter, for distances of from 400 to 800 feet to a centrifugal machineoperated well beyond reach inside the water main. The potential fordisaster in the event of unforeseen delays or of unpredictable set timewould make even the conservative use of accelerators risky. Theseaccelerators are subject to wide variations in time required for set andare therefore not dependable in field circumstances where widetemperature and material variations are probable. Second, even thoughthe mortar quickly attains initial set through the use of accelerators,it is still vulnerable to surface scouring by a flow of water until itbecomes harder. This would require an unacceptable delay in returning awater main to service.

In addition to the above problems of achieving a rapid set time so thatout-of-service time can be reduced, there remains the problem of thepreviously discussed release of high pH constituents into the water of awater main when it is first returned to service thereby reducing waterquality.

U.S. Pat. No. 2,917,778 to Lyon Jr. et al, 4,362,679 to Malinowski,4,093,690 to Murray, and 4,746,481 to Schmidt disclose processes forcuring various manufactured articles or, in the case of the castingprocess of Malinowski, floors, walls, and the like on a building site,by the use of carbon dioxide in molds, presses, and the like. The LyonJr. et al process involves the closed vessel curing of concrete linedvessels such as water heater shells, and the patent states, at Col. 2,lines 25 to 28, that it is sufficient that the carbon dioxide in thetank be enough to neutralize the layer of calcium hydroxide which is onthe surface of the concrete. The time period discussed is undesirably onthe order of several days (in Example I, it was 28 days).

While Malinowski, Murray, and Schmidt discuss short curing times ("5 to30 minutes" in the Schmidt press, and "a few minutes" in the Murraychamber), they do not teach or suggest stopping a reaction of carbondioxide with deposited cement mortar before a crust of calcium carbonatehas been formed through the entire thickness of the cement mortardeposit, and such a crust has a low pH.

Other patents which may be of interest include Japanese patent document114,823, and U.S. Pat. Nos. 825,088; 898,703; 1,932,150; 2,363,226;3,249,665; 3,358,342; 4,252,763; 4,350,567; 4,427,610; 4,436,498;4,772,439; and 5,051,217.

By providing a crust of calcium carbonate, high pH constituents may beprevented from release into the water so that the water quality is notreduced, as previously discussed. However, in using carbon dioxide gasto form the crust, it is important that the water main be returned toservice quickly and that the layer of mortar adjacent the pipe maintaina high pH for corrosion-resistance at the pipe interface. It is alsodesirable that the crust be formed dependably and efficiently.

OBJECTS AND SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide adependable and efficient method and apparatus for lining water mainswherein service may be more quickly restored and wherein direct andindirect expenses associated with delays and the requirement forattendant aboveground temporary piping may be reduced.

It is another object of the present invention to maintain a high pH inthe layer of mortar adjacent the pipe for corrosion-resistance at thepipe interface.

It is yet another object of the present invention to prevent or reducethe release of high pH constituents into the water of the lined watermain when it is first returned to service.

In order to efficiently line a water main with a minimum ofout-of-service time, in accordance with the present invention the freshmortar lining is exposed to a minimum positive pressure of carbondioxide gas for a period of time which allows it to combine chemicallywith calcium compounds of the wet cement mortar to form a crust ofcalcium carbonate which will tolerate a low-velocity flow of water andprotect the underlying mortar from scouring until the normal hardeningof the mortar through hydration of the cement can take place.

The foregoing and other objects, features, and advantages of the presentinvention will be more fully understood after a consideration of thefollowing detailed description taken in conjunction with theaccompanying drawings wherein the same reference numerals denote thesame or similar parts throughout the several views and in which apreferred embodiment of the present invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generally diagrammatic view of a method of lining a sectionof pipe in accordance with the prior art.

FIG. 2 is a generally diagrammatic view illustrating the lining of asection of pipe in accordance with the present invention.

FIG. 3 is a sectional view of the section of pipe of FIG. 2 after liningthereof and in condition for water flow therethrough.

FIG. 4 is an enlarged view similar to that of FIG. 3 of the portion ofthe section of pipe circled in FIG. 3.

DETAILED DESCRIPTION

Referring to FIG. 2, there is illustrated a method, in accordance withthe present invention, of lining a section of water main 10 bydelivering wet mortar from a mixer, which may be similar to mixer 23, tolining apparatus 24, and throwing the wet cement mortar 36 onto thecleaned surface of the water main 10, similarly as described for themethod illustrated in FIG. 1. Squeegee or rubber swab 19 is also pulledthrough the pipe section to complete the cleaning process beforeapplication of the mortar. The apparatus 24 is winched through the line10 by operation of a winch, similar to winch 30, with winch line 34. Forreduced cost, the trowel, illustrated at 32 in FIG. 1, may, if desired,be eliminated, as indicated by its absence in FIG. 2.

As previously indicated, the prior art illustrated in FIG. 1disadvantageously requires that residential and commercial businesslines be provided by temporary service connections since the water mainto be lined will be out-of-service for typically several days during thelining and subsequent hardening of the wet cement mortar by normalhydration. A conventionally applied lining will normally develop acalcium carbonate layer from interaction with carbon dioxide in thewater over a long period of perhaps 5 to 10 years. This layer isnaturally retarded from extending through the entire thickness so thatthe layer remains relatively thin.

In accordance with the present invention, a calcium carbonate crust isformed quickly by the use of carbon dioxide gas in order that the watermain pipe may be put back in service quickly. Thus, as illustrated inFIGS. 2 to 4, in accordance with the present invention a relatively highconcentration of carbon dioxide gas, illustrated at 84, from pressurizedtank 47 is introduced into the freshly lined water main in sufficientquantities to form a crust, illustrated at 80, of calcium carbonate onthe interior surface of the wet cement mortar, illustrated at 36,sufficiently quickly and to sufficient depth as to reduce theout-of-service time of the water main to an acceptable level, i.e.,perhaps only for 7 hours or less. This will reduce the delay inrestoring service as well as the costs associated with providingtemporary service connections. The carbon dioxide gas tank 47 has asuitable valve 46 connected to its outlet for controlling the flow ofcarbon dioxide gas 84 from tank 47 to the interior of the pipe section10 being lined.

In accordance with the present invention, an airtight bulkhead or cap 38is mounted to an end (farthest from the winch) of the section of thepipe which is being provided with a wet cement mortar 36. The cap 38 hasan opening to which is connected one end of a portion of feed hose 50the other end of which is connected to bellows 60. The purpose of thebellows will be described later. Another portion of the feed hose 50 issuitably connected to and extends between bellows 60 and the controlvalve 46. The portion of hose 50 between the cap 38 and bellows 60 ispreferably of sufficiently large diameter, perhaps 1 to 11/2 inch, toprevent flow rate reduction due to freezing associated with expansion ofthe compressed carbon dioxide gas from high pressure to atmosphericpressure. Thus, carbon dioxide gas is suitably released from tank 47into the water main section interior to react with the mortar, aspreviously discussed.

The tight fitting rubber swab 19 is provided on lining machine 24 notonly as a means for removal of standing water before mortar applicationbut also to displace air as the lining machine is pulled through thepipe section during the lining operation and to draw in carbon dioxidegas 84 from tank 47.

In accordance with alternative embodiments of the present invention, atrowel, similar to trowel 32 of FIG. 1, or other suitable means may beplaced either ahead of the pipe lining machine instead of the swab 19 orafter the pipe lining machine to perform the functions the swab wouldperform including air displacement.

After completion of lining and removal of the lining machine 24, theother end of the pipe section is preferably capped by airtight bulkhead51 which has an opening to which is suitably connected a small diameterhose 40 the end of which is closed by a suitable control valve 52.

Since carbon dioxide gas is heavier than air, tank 47, valve 52, andbellows 60 are preferably positioned at ground level or otherwise wellabove the pipe 10.

The calcium carbonate layer which is formed by the process of thepresent invention would naturally be formed anyway to a certain depthover a period of time. However, the crust formed by the application ofthe carbon dioxide gas is not retarded from extending through the entirelining thickness in the same way that the naturally formed layer isretarded. Thus, the carbon dioxide gas 84, over a period of perhaps 2 or3 days, may penetrate all the way through the lining thickness so thatthe crust would extend through the entire lining thickness.

Carbon dioxide lowers the mortar pH so that the crust is more acidic andtherefore less effective for inhibiting corrosion. It is thus importantthat the layer of mortar adjacent the pipe not be converted to calciumcarbonate.

By returning the water main section to service within about 7 hoursthereby displacing the carbon dioxide gas in the section, the reactionis ceased so that the mortar pH at the pipe interface remains desirablyhigh. Thus, by controlled use of a reaction which naturally occurs overa period of time anyway, the water main can be put back in servicequickly for the convenience of the customers while maintainingcorrosion-resistance at the pipe interface. This crust 80 should notlessen the value of the lining because the formation of a calciumcarbonate layer would naturally occur anyway.

In order to effect formation of the crust, a positive carbon dioxide gaspressure, i.e., a pressure higher than atmospheric pressure, is desiredto maintain saturation. However, pressures higher than required tomaintain a saturated condition may not provide additional benefit incrust formation. Moreover, excess carbon dioxide gas constitutes a wastewhich not only increases costs but also may undesirably be released tothe atmosphere. Further, excess pressure may damage the swab and mayresult in freezing associated with expansion of the compressed carbondioxide gas. Thus, in accordance with the present invention, a pressurewithin the water main section which is only slightly higher thanatmospheric pressure, i.e., a minimum positive pressure, is maintainedto effectively and quickly form the crust 80 with a minimum of waste ofcarbon dioxide gas 84. As the air is displaced from the pipeline 10, itis only necessary that a vacuum be prevented to insure filling of thepipeline with the carbon dioxide gas 84. Too great of pressure, inaddition to wasting carbon dioxide gas, may also exert an undesirablyhigh force on the swab 19. Thus, the desired carbon dioxide pressure isgreater than atmospheric pressure but not over about 1 psig (pounds persquare inch gage), more preferably, not over about 0.1 psig. Forexample, a pressure of about 0.01 psig is considered desirable.Conventional pressure gages may not have the capability of measuringsuch low pressures to indicate such low positive pressures withsufficient precision. As used herein and in the claims, the term"minimum positive pressure" is meant to refer to a pressure greater thanatmospheric pressure and less than about 1 psig.

In order to precisely regulate the carbon dioxide gas pressure withinthe water main section to maintain the desired positive pressure, thebellows, illustrated at 60, or other suitable means for providing ameasurement of pressure in the 0 to 1 psig range is provided in the gasinlet line 50. The bellows may be characterized as a thinaccordion-shaped membrane 62, within a suitable container 64, whichmembrane unfolds and thus expands by a large amount with small increasesin pressure and contracts by a large amount with small decreases inpressure, the pressure within the line 50 and accordingly within thewater main section being supplied to the interior of the bellowsmembrane 62 in accordance with principles commonly known to those ofordinary skill in the art to which this invention pertains. An exampleof a suitable bellows is an ATL flexible bladder marketed by Aero TecLaboratories Inc. of Ramsey, N.J. The bellows 60 may suitably have anindicator rod 68 and a pointer 66 which is suitably connected theretoand which is calibrated relative to bellows expansion and contraction toprovide a precise indication of slight positive pressure. The bellows 60may be said to be a variable volume enclosure in pressure communicationwith the interior of the pipe line. Thus, the bellows 60 may be said tomeasure excess volume for the available carbon dioxide gas 84 within thechanging volume within the pipeline 10 and line 50 at any given time asthe pipe-lining machine 24 is moved through the pipeline. An excessvolume measurement is an indicator of positive pressure which may bequantified by pointer 66. The continuous positive pressure obtainable byuse of the bellows and the sealing provided by the swab are thuslyprovided to achieve a concentration of 100% carbon dioxide continuouslyas the volume is changing due to movement of the swab.

A preferred process for lining a section of water main 10 is as follows.Begin lining with swab 19 just ahead of lining machine 24 and with themachine 24 applying a suitable cement mortar coating 36 to the interiorsurface of the pipe 10. The machine 24 is withdrawn at a rate that iscalculated to deposit a lining of a thickness which preferably is keptto a minimum consistent with providing a sufficient thickness. Thethickness is minimized so that the time of application may be reducedand the thickness of compressible uncured mortar under the hard crustwill be small to help minimize damage. A mortar thickness, illustratedat 82, of, for example, about 0.1 inch (2.5 mm) is considered to besatisfactory.

As soon as the lining machine 24 has fully entered the pipe 10, thebulkhead 38 is securely attached to the pipe end. Valve 46 is opened andsuitably adjusted to provide a flow of carbon dioxide gas 84 into thepipe interior at a suitable rate of perhaps about 11 cubic feet perminute to provide a minimum positive carbon dioxide pressure of perhapsabout 0.01 psig. The flow rate of carbon dioxide gas 84 is suitablyadjusted such as by valve 46 to maintain the desired pressure as liningmachine 24 recedes. The pressure is indicated by pointer 66 of bellows60. If desired, the valve 46 may be suitably connected to bellows 60 forautomatic control of the pressure.

The machine 24 is receded at a suitable rate such that, for example, itwill emerge from the other end of a 150 meter long section of pipeapproximately 17 minutes later. The bulkhead 51 is then immediatelyfixed to the end of the pipe section from which machine 24 is withdrawn.The valve 52 may be opened briefly to vent any air from the pipeline,then closed and valve 46 used to maintain the desired positive pressureuntil the crust forms. If desired or needed, a carbon dioxide detectormay be provided to determine that the pipeline is suitably vented. If itis desired or necessary to insure a slight positive pressure throughoutthe water main section length, another bellows, similar to bellows 60,may be provided in outlet line 40.

As carbon dioxide gas is consumed in forming the crust, more must beadded to maintain saturation. Thus, a very small flow rate may bemaintained. The carbon dioxide gas at a slight positive pressure is thenallowed to react with the cement mortar 36 for a suitable period ofperhaps 1 hour perhaps while the other lining equipment is cleaned.

After the period of reaction time, the valve 46 is closed, the bulkheads38 and 51 are removed, and the pipe spools are re-installed to completethe pipe 10. Mainline valves may then be opened slightly to allow waterto flow slowly into the newly lined pipe 10, which may thereafter berestored to service after suitable disinfection, thereby displacing thecarbon dioxide gas in the section of pipe to stop the reaction of thecarbon dioxide with the deposited cement mortar before the crust 80 ofcalcium carbonate has been formed through the entire thickness of thecement mortar deposit.

The above process is provided so that a sufficiently hard crust 80 maybe formed quickly, perhaps in about 1 hour, on the lining after placingso that the pipeline may receive water without erosion damage untilnormal hardening of the mortar through hydration of the cement can takeplace. The carbon dioxide gas 84 combines chemically with the cementclose to the surface of the mortar to produce calcium carbonate, whichforms the protective crust to a depth, illustrated at 86, of perhapsabout 0.4 mm over the soft mortar. This allows the pipe to accept theintroduction of water without damage and without the undesirable releaseof high pH constituents into the water while the underlying soft mortarcontinues to harden normally. This allows the re-introduction of waterinto a water main within perhaps 2 hours of lining thereof so thatrehabilitation of the length of main may be completed within a workingday. Yet the value of the lining is not lessened since a calciumcarbonate layer would naturally form anyway. Thus, the out-of-servicetime may be substantially reduced to perhaps 7 hours or less for a moreacceptable disruption of service to customers, and the added expense ofproviding temporary water service lines may be eliminated.

While a preferred lining process is described above, it should beunderstood that other suitable lining processes may be employed to reactthe carbon dioxide gas with the mortar. For example, the mortar may beapplied to an entire section which is then inspected to insure that itis suitably placed before the introduction of carbon dioxide gas. As thecarbon dioxide gas 84 is introduced, the valve 52 is opened to vent theair from the pipeline (or the pipeline otherwise suitably vented), and asuitable carbon dioxide detector (which may measure lack of oxygen) usedto indicate that the air in the pipeline has been displaced by carbondioxide gas. The valve 52 may then be closed and the valve 46 adjusted,as previously discussed, to maintain slight positive pressure forformation of the crust. Such other processes are meant to come withinthe scope of the present invention.

For purposes of illustration and not for limitation, the followingexemplary calculation is provided. Type II Portland cement is used, inaccordance with the example, for the cement mortar lining because itsslower setting time allows it to be conveyed and applied with greatersafety. This cement is composed of, by weight, 42 percent 3CaO.SiO₂, 33percent 2CaO.SiO₂, 5 percent 3CaO.Al₂ O₃, and 13 percent 4CaO.Al₂ O₃.Fe₂O₃. As water is added to the cement, calcium hydroxide is immediatelyformed, the excess water continuing to hydrate the cement over a longperiod. The pipe section to be lined has an inner diameter of 200 mm anda length of 150 meters providing a surface area to be lined of about94.25 square meters. For a typical crust thickness of 0.4 mm, the volumeof crust is calculated to be about 0.0377 cubic meters. It is known thatthe density of the cement mortar, which has 1 part sand to 1 part cementto 0.4 part water, is about 2200 kg per cubic meter. From this theweight of the cement in the crust is calculated to be about 34.6 kg.When water is added to the cement, the total weight of calcium hydroxideformed is calculated to be about 14.4 kg. The calcium hydroxide combineswith the carbon dioxide gas according to the equation:

    Ca(OH).sub.2 +CO.sub.2 →CaCO.sub.3 +H.sub.2 O

The molecular weights of calcium hydroxide and carbon dioxide are 38 and28 respectively providing a weight ratio of carbon dioxide to calciumhydroxide of 28/38 or 0.74. Therefore, the weight of carbon dioxide gasrequired to produce the hardened crust 0.4 mm thick on 150 meters of 200mm inner diameter pipe is 0.74×14.4 kg or 10.6 kg (23.3 lbs.). Theweight of carbon dioxide gas at standard temperature and pressure thatis required to fill the pipe section is 27.7 kg. (61 lbs.) which isgreater than the amount required for the reaction. Thus, the maintenanceof a slight positive pressure of perhaps 0.01 psig should insure morethan enough carbon dioxide gas to complete the chemical reaction, whichmay take perhaps 1 hour to reach the desired depth after which waterflow is reestablished to displace the carbon dioxide gas so as to stopthe reaction at the desired crust depth. The carbon dioxide gaspressure, as indicated by bellows 60, is adjusted by manipulating valve

It is to be understood that the present, invention is by no meanslimited to the specific embodiments which have been illustrated anddescribed herein and that various modifications thereof may indeed bemade which come within the scope of the present invention as defined inthe appended claims.

What is claimed is:
 1. A method of lining a section of water main pipehaving an interior surface comprising taking the water main pipe sectionout-of-service, depositing a cement mortar on the interior surface toform a lining, forming on an interior surface of the lining of cementmortar a crust of calcium carbonate to a thickness which is less than anentire thickness of the cement mortar lining such that some cementmortar underlies the crust for protection by the crust of calciumcarbonate of the underlying cement mortar from scouring as water isintroduced into the section of pipe until hardening of the underlyingcement mortar through hydration of the cement mortar takes place, thestep of forming the crust of calcium carbonate comprising introducingcarbon dioxide gas into the section of pipe to react with the depositedcement mortar until the crust of calcium carbonate is formed, regulatingthe introduction of the carbon dioxide gas to maintain a minimumpositive pressure of the carbon dioxide gas within the water main pipesection, and returning the water main pipe section to service therebydisplacing carbon dioxide gas in the section of pipe to stop thereaction of carbon dioxide gas with the deposited cement mortar beforethe crust of calcium carbonate has been formed through the entirethickness of the cement mortar lining.
 2. A method according to claim 1further comprising regulating the introduction of carbon dioxide gas sothat the minimum positive pressure of carbon dioxide gas within thesection of pipe is less than about 0.1 psig.
 3. A method according toclaim 1 further comprising providing a bellows for indicating thepositive pressure of the carbon dioxide gas within the section of pipe.4. A method according to claim 1 further comprising returning the watermain pipe section to service within about 7 hours after the pipe sectionis taken out of service.
 5. A method according to claim 1 furthercomprising regulating the introduction of carbon dioxide gas so that theminimum positive pressure of carbon dioxide gas within the section ofpipe is equal to about 0.01 psig.
 6. A method according to claim 1further comprising sealing a first end of the water main pipe section,introducing the carbon dioxide gas at the sealed first end of the watermain pipe section, drawing a pipe lining machine through the water mainpipe section away from the first end while depositing the cement mortar,providing on the pipe lining machine a seal for sealing the water mainpipe section as the pipe lining machine is drawn therethrough, andintroducing the carbon dioxide gas as the pipe lining machine is drawnthrough the water main pipe section.
 7. A method according to claim 6further comprising removing the pipe lining machine from a second end ofthe water main pipe section after the cement mortar has been deposited,sealing the second end of the water main pipe section after the pipelining machine has been removed therefrom, and regulating theintroduction of carbon dioxide gas to maintain the minimum positivepressure thereof in the water main pipe. section until the crust ofcalcium carbonate is formed.